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Home > Human Impact on the Reef > Polution and Other Factors 


5(e) Pollution and Other Factors

Other Factors Causing Coral Bleaching: There are other factors, apart from heat stress, that cause coral bleaching. These may include:

  • increased exposure to UV radiation
  • increased storm water run-off after heavy rains
  • certain chemicals or diseases
  • increased sediments such as sand and mud covering the coral
  • excess nutrients from ammonia and nitrate-based fertilisers and other household products, entering the reef ecosystem.

Nutrient Enrichment: Nutrient enrichment on the GBR is associated with:

  • coastal development and runoff leading to poor water quality
  • fertiliser run-off from farming and agricultural practices
  • storms which cause re-suspension of nutrients.

This increase in nutrients in aquatic ecosystems that results in increased algal growth is called eutrophication. Large amounts of algae can block the pores on sponges, block light filtering through to the zooxanthellae living within some corals, and decrease a coral’s resistance to disease. This may also lead to extremes in the productivity of plankton, more variable survival of larvae and recruitment of fish populations, and overgrowth of coral reef organisms by algae.

Other Risks to the Reef: Further risks to coral reefs from human activities include:

  • the impact of anchor damage
  • unsustainable fishing and harvesting practices
  • deep sea dredging and trawling
  • cyclonic weather
  • population explosions of coral-feeding organisms (e.g. Crown of Thorns starfish)
  • coral diseases.

Effects on Other Species: Nine percent of fish on the GBR are directly coral-dependent for food, habitat, or both. Fish that are directly affected by the loss of living coral are the butterfly fish, damselfish and the coral goby. Butterfly fish such as Chaetodon trifascialis, are coralivores, that is, animals for which live coral is their main food source.  However, it’s not just these fish that are affected. Approximately 65% of fish species preferentially settle in or near live coral, and a huge number of invertebrate species obligatorily depend on the coral for their survival.

Butterfly Fish
Butterfly Fish. Image: Leonard Low, Creative Commons.

Over-fishing is another cause for concern because when parrotfish and large herbivorous fish are removed from reef food webs, it allows an increase in Sargassum algae and cyanobacteria that form large beds over the coral. This blocks off the light to the zooxanthellae algae and affects the resilience of corals. That is, it affects their ability to recover from coral bleaching events.

You may like to design and perform an investigation, ‘Photosynthesis in Algae’, and develop your scientific thinking skills by doing the structured discussion on “What is the effect of overfishing on the Great Barrier Reef?’ in the For Teachers section.

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